1. Field of the Invention
The invention relates to a method of breaking optical fibers destined for optical communication systems, a fiber to be broken being fixed in position, while subsequently the circumference of the fiber is scored with the aid of a scoring element, after which a predetermined axial tensile force is applied to the fiber and finally the fiber is broken at the location of the score.
2. Description of the Prior Art
Glass fibers destined for optical communication systems must comply with special requirements, so as to minimize the loss at the coupling between laser-fiber, fiber-fiber, and fiber-optical receiver. For example, the end faces of the fiber should have a surface of optical surface quality; furthermore, the end faces, in particular those of monomode fibers, should be perpendicular to the fiber axes to the highest possible degree.
Glass rods and glass fibers, scored and broken in accordance with one of the conventional methods, exhibit a characteristic morphology at the fracture surface, with a so-called mirror zone, mist zone and hackle zone; the mirror zone is a fracture surface portion of optical surface quality adjacent to the score; the hackle zone is a fracture surface portion where the fracture has separated the specimen into at least three distinct pieces; the mist zone is a fracture surface portion formed by a transition between the mirror zone and the hackle zone. This phenomenon is described comprehensively in the article by Johnson and Holloway: "On the Shape and Size of the Fracture Zones on Glass Fracture Surfaces" published in the British magazine "Philisophical Magazine", No. 14, October 1966, pages 731 to 743.
For most applications in optical communication systems the entire fracture surface of the fiber must be constituted by a mirror zone, while in many cases stringent requirements are imposed on the perpendicular circulation of the fracture surface relative to the fibre axis.
From the previously cited publication a simple, empirical equation is known, which relates the shape and size of the mirror zone to the stress distribution over the cross-section of a fiber before the initiation of fracture. For all points P of the mirror zone: EQU Z.sub.p r.sup.1/2 =C[N/mm.sup.3/2 ]
where Z.sub.p is the force component normal to the fracture plane of the local stress at the point P before fracture begins; r is the distance from the origin of fracture to the point P; C is a material constant.
For example for lead glass C has a value of approx. 5.5, for lime glass 6.0 and for quartz glass 6.5.
In order to obtain a fracture surface having only a mirror zone, each point P on the fracture surface should meet the requirement: EQU Z.sub.p .multidot.r.sup.1/2 &lt;C[N/mm.sup.3/2 ]
An other limiting factor is that the value of Z.sub.p for an arbitrary point P should not decrease to zero or even become negative, because otherwise fracture will continue in a direction which is not perpendicular to the fiber axis. In that case a so-called lip may be produced at one of the fiber ends.
A method in accordance with the preamble is known from the article by Gloge et al: "Optical Fiber End Preparation for Low-Loss Splices" published in the American journal "The Bell System Technical Journal", Vol. 52, Nr. 9, November 1973, pages 1579 to 1588. In accordance with this known method a specific decreasing stress distribution over the cross-section of the fiber is obtained by bending the fiber over a convex surface in order to obtain a low value of Zp r.sup.1/2 and thus to ensure that solely a mirror zone is produced on the fracture surface; fracture is then initiated by scoring the bent portion of the fiber at the location of maximum stress.
A drawback of this known method is that the perpendicular orientation of the fracture surface may be unsatisfactory and that a comparatively high stress of the order of magnitude of 250 N/mm.sup.2 is required to initiate fracture. Moreover, the scoring element must be manipulated with care so as to minimize the disturbed area in the vicinity of the score.